1. Field of the Invention
The present invention relates to an optical transmission device having an optical amplifier in a WDM (Wavelength Division Multiplexing) apparatus.
2. Description of the Related Art
FIGS. 1A and 1B explain a conventional technique.
Normally, a noise figure (hereinafter abbreviated to an NF) of an optical amplifier for WDM tends to increase with an increase in the input level of the amplifier if its output is made constant. FIG. 1A shows the state of an increase in the NF of an optical amplifier module. Pin is an input level of light input to the optical amplifier module, and indicates an optical intensity per channel. This figure indicates that the NF nonlinearly increases with an increase in Pin over the entire input dynamic range of the optical amplifier module. In the meantime, FIG. 1B indicates a relationship between Pin and an OSNR (Optical Signal-to-Noise Ratio). The OSNR exhibits an upward convex curve with an increase in Pin, and a point where the OSNR becomes a maximum exists. In the following description, an optical amplifier unit is a configuration of an entire optical amplifier module including an optical amplifier module, a control circuit adjusting the gain of the optical amplifier module, etc. Additionally, the optical amplifier module is part of an optical amplifier unit including a minimum configuration that is required to amplify an optical signal, and includes at least an optical amplification medium and a pumping light source.
Furthermore, since the wavelength dependence of a gain relies on the gain of an optical amplifier in a WDM transmission, the gain must be made constant. Eventually, it is difficult both to maintain a low NF, namely, to suppress occurring noise and reduce wavelength dependence, and to widely secure the input dynamic range of the optical amplifier.
Normally, spans having different losses coexist in an optical transmission system using optical amplifiers. Therefore, the input of an optical amplification relay can possibly take various values, and input level a wide range. Accordingly, to maximize the performance of the system, input levels in a wide range must be supported by preparing a plurality of types of optical amplifiers having input dynamic ranges of different levels, and by selecting an optimum type of an optical amplifier according to an input level. Furthermore, a span loss varies due to fluctuations in a temperature, etc. Besides, a span loss may vary due to a problem transfer (a line change for coping with a road construction, etc.), or the like. In such a case, a type of an optical amplifier must be changed, and a new component must be provided. On an already operated route, a type of an optical amplifier cannot be changed unless the operation is suspended.
As a method supporting a span loss in a wide range, a method adjusting an input level of an optical amplifier module to fall within the input dynamic range of the amplifier is considered. As a method adjusting an optical level, a method arranging a variable attenuator (hereinafter abbreviated to a variable ATT) prior to an optical amplifier module, combining the variable ATT with a monitor coupler for measuring an input level of an optical amplifier module, and with a PD monitor, and adjusting the optical level in order to make the input level of the module constant is considered.
FIG. 2 exemplifies the configuration of a conventional optical amplifier unit.
An optical amplifier unit 10 comprises: in addition to an optical amplifier module 16, a variable attenuator (variable ATT) 11 for adjusting the optical level of an input to the optical amplifier module 16; an optical coupler 12 for splitting and detecting OSC (Optical Supervisory Channel) light; an opto-electric converter 13; an optical coupler 14 and a PD monitor 15 for monitoring the input level of light input to the optical amplifier module 16; an optical coupler 17 and a PD monitor 18 for monitoring the output level of the optical amplifier module 16; an electro-optic converter 20 and an optical coupler 19 for coupling the OSC light to a main signal; and a control circuit 21 controlling the variable ATT 11.
In FIG. 2, the variable ATT 11, and the two optical couplers 12 and 14 exist between the input (UPin) of the optical amplifier unit 10 and the input (Pin) of the optical amplifier module 16, and losses in these optical components affect the OSNR (Optical Signal-to-Noise Ratio): optical N/S ratio) of the optical amplifier unit 10. The NF characteristic of the optical amplifier module/unit, and the OSNR characteristic of the optical amplifier module/unit are respectively shown in FIGS. 3A and 3B. If UPin exists in a region (A) of FIG. 3B, the gradient of the NF of the optical amplifier unit becomes approximately 0 dB/dB. At this time, the OSNR has a gradient of 1 dB/dB. If UPin decreases due to a loss, also the OSNR decreases by the amount of the decrease of UPin. Namely, since the loss and the OSNR make a one-to-one correspondence, losses in the above described optical components result in a deterioration in the OSNR. Accordingly, decreasing the losses as much as possible is required to improve the OSNR.
As conventional techniques, Patent Documents 1 and 2 exist. Patent Document 1 discloses a technique changing an optical input level in order to make an input level to an optical amplifier constant if an optical signal is disconnected. Patent Document 2 discloses a technique implementing a redundant configuration where hardware is shared by using an optical coupler whose branch ratio can be varied.
[Patent Document 1] Japanese Patent Application Publication No. 2000-312185
[Patent Document 2] Japanese Patent Application Publication No. 2001-339344
As described above, the OSNR of an output signal of the optical amplifier unit deteriorates due to losses in the optical components within the optical amplifier unit. Therefore, reducing the losses in the optical components as much as possible is effective if attempts are made to fully utilize the performance of the optical amplifier module.